Variable speed, constant-frequency power generating systems are often used to convert variable-speed motive power produced by a prime mover into constant-frequency AC power. Typically, such systems include a brushless, synchronous generator which converts the variable-speed motive power into variable-frequency electrical power, a rectifier which converts the variable-frequency electrical power into DC power and a polyphase inverter which converts the DC power into the constant-frequency AC power. The inverter may be of the stepped-waveform type wherein a series of polyphase subinverters are interconnected by a summing transformer which in turn produces polyphase stepped-waveforms each approximating a sinusoidal output.
Occasionally, an unbalanced condition can arise in loads connected to the inverter output. This, in turn can cause the output voltage magnitudes of the summing transformer to become unbalanced or substantially unequal. Such a condition is undesirable where output voltage magnitudes must be held within a certain range of a specified level.
A 24-step waveform inverter having a summing transformer which sums the outputs of four subinverters is disclosed in Compoly et al., U.S. Pat. No. 3,775,662.
A voltage balancing circuit for a polyphase inverter is disclosed in Mehl, co-pending application Ser. No. 607,811, filed Nov. 1, 1990 entitled "Voltage Balancing Circuit" and assigned to the assignee of the instant application. The voltage balancing circuit includes a pair of supplemental inverters producing supplemental AC power which is combined with the output power on two of three output phases of the inverter so that all of the phase voltage magnitudes are maintained within a range of voltage magnitudes.